29 JIT Compilation Interface

This chapter documents GDB’s just-in-time (JIT) compilation
interface. A JIT compiler is a program or library that generates native
executable code at runtime and executes it, usually in order to achieve good
performance while maintaining platform independence.

Programs that use JIT compilation are normally difficult to debug because
portions of their code are generated at runtime, instead of being loaded from
object files, which is where GDB normally finds the program’s symbols
and debug information. In order to debug programs that use JIT compilation,
GDB has an interface that allows the program to register in-memory
symbol files with GDB at runtime.

If you are using GDB to debug a program that uses this interface, then
it should work transparently so long as you have not stripped the binary. If
you are developing a JIT compiler, then the interface is documented in the rest
of this chapter. At this time, the only known client of this interface is the
LLVM JIT.

Broadly speaking, the JIT interface mirrors the dynamic loader interface. The
JIT compiler communicates with GDB by writing data into a global
variable and calling a fuction at a well-known symbol. When GDB
attaches, it reads a linked list of symbol files from the global variable to
find existing code, and puts a breakpoint in the function so that it can find
out about additional code.